This invention relates to electrostatic actuators.
Electrostatic actuators are used in micro-electromechanical systems (MEMS) for producing fine positional adjustments of a load along one axis or multiple axes.
A typical electrostatic actuator includes multiple pairs of electrode elements spaced by a gap and mounted on two plates. One plate is stationary, while the other plate is mounted for movement with respect to the first plate and is attached to the load. When energy is applied to the plates, the voltage induced across the gaps between the pairs of elements generates an electrostatic attractive force between the pairs of electrodes, which in turn produces relative motion between the plates, thereby moving the load.
In one general aspect, this invention features placing a liquid in the gap between elements of an electrostatic actuator.
Preferred embodiments may include one or more of the following features.
The liquid has a dielectric constant selected to produce a predetermined increase in an electrostatic force produced between the elements in response to a given applied voltage. Such force amplification is produced if the dielectric constant exceeds one. Thus, the dielectric constant of the liquid is preferably greater than one, more preferably is greater than 2, and most preferably is greater than 4.
The liquid is substantially nonelectrically conductive. For example, the liquid has a resistivity of on the order of 1012 ohm-cm. or higher. Such low conductivity avoid corrosive electrolytic interaction between the elements which could degrade the life of the actuator.
The liquid is chemically stable and has a high breakdown so as not to substantially dissociate in the presence of an electric field produced between the elements by the applied voltage. This avoids the liquid becoming progressively more electrically conductive over time. The chemical stability also inhibits electrical breakdown between the actuator elements.
The viscosity of the liquid provides a selected amount of damping of the relative movement of the elements. Preferably, the viscosity is less than 1 poise, and more preferably the viscosity is less than 0.01 poise.
The liquid has a low vapor pressure to avoid the liquid evaporating away during fabrication of the actuator or subsequent use. If the vapor pressure is selected to be sufficiently low, e.g., on the order of 10xe2x88x927 mm Hg or less, may avoid the need for a containment envelope for the liquid. Preferably, the liquid has a melting point of less than 0 degrees F. to remain in the liquid state over a normal range of operating temperatures.
One exemplary liquid is diffusion pump oil. Other examples of the liquid include perfluorcarbons and hydrofluorocarbons, and halides. The liquid may be selected from a group consisting of chlorobenzene, cis 1,2 dichloroethylene, orthodichlorobenzen, and nitrobenzene.
The actuator may include an envelope in which the elements and the liquid are disposed. Because it is contained and cannot evaporate, the liquid may have a lower vapor pressure than the range given above. The actuator may also include a biasing member coupled between the elements to return the elements to an initial relative position when the voltage is removed therefrom.
In a particularly useful application, the actuator is used to position a transducer which exchanges data signals with a storage medium in a data storage apparatus. The first member is connected to an arm and the second member includes a support for the transducer and is mounted for movement with respect to the arm. The first element of the actuator is part of a first group of elements mounted on the first member, and the second element is part of a second group of the elements mounted to the second member. The liquid is disposed in gaps between corresponding elements in the first and second groups.
In one embodiment, the elements in the first and second groups are linearly disposed in a first direction with respect to the arm and the transducer support and are interleaved with each other, whereby the relative movement is linear and transverse to the first direction. In another embodiment, the elements in the first and second groups are disposed in a circular pattern about an axis and are interleaved with each other, whereby the relative movement is rotational about the axis.
The transducer may be, e.g., a magnetic sensor, a giant magnetoresistive head, or an optical sensor.
The liquid provides numerous advantages. The force amplification increases the strength of the actuator and enables the actuator to be used in a variety of applications. In addition, the liquid avoids contact between the elements that might otherwise occur during use. This helps prevent arcing between the elements and accompanying wear, as well as catastrophic failure such as the plates breaking as they collide.
Other features and advantages of the invention will be apparent from the following detailed description, and from the claims.